This application is based on applications Nos. 10-188724/1998, 10-188730/1998 and 10-188736/1998 filed in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus which forms a color image by superposing images of a plurality of colors formed by a plurality of print units.
2. Description of Prior Art
In a color image forming apparatus of tandem structure, an image of four colors is formed at the same time in a scan. In the color image forming apparatus, four printer units including photoconductors for the four print colors are aligned, and images of the four colors are supplied to exposure positions for the colors, by delaying in correspondence to the distances between the photoconductors. In such an apparatus of tandem structure, print positions of the color images are liable to be shifted, due to delay of image data through memories, shifts of exposure position on the four photoconductors and the like. Then, a color shift may arise in an image to cause image distortion such as bow distortion.
Then, in order to produce an image of high quality, it is desirable to correct image distortion. It is proposed to detect image distortion and to change line address for each color in the subscan direction in correspondence to an address in the main scan direction based on the detected image distortion. However, in this case, correction can be performed in the unit of address, but not in the unit of a smaller size. Further, because the correction in the main scan direction is not performed, management of correction data is complicated for performing delicate correction.
Further, in the above proposal, if the detected image distortion exceeds the maximum image distortion (or line number), image forming is inhibited. Then, printing may be inhibited frequently.
An object of the present invention is to provide an image forming apparatus having an image distortion correction system for forming a color image.
In one aspect of the invention, an image forming apparatus comprises an image forming unit having a plurality of print units each forming an image of one of a plurality of colors based on image data. A pattern generator generates image data of a predetermined pattern for the plurality of colors. A sensor detects images of the predetermined pattern of the plurality of colors formed by the print units according to the image data generated by the pattern generator, while a detector detects a setting error of a position of the sensor. A controller calculates correction data based on the setting error detected by the detector and image distortion determined from detection data detected by the sensor.
In another aspect of the invention, an image forming apparatus comprises a printer which forms an image of a frame by forming an image for each line according to image data in the unit of pixel. A sensor detects a quantity of image distortion in the image generated by the printer during printing operation. First and second memories store la first component in a direction perpendicular to a line direction and a second component in the line direction, respectively, of the quantity of image distortion detected by the sensor for each pixel address in the line direction. A corrector corrects the image distortion by providing print data of a pixel at a position corrected in the line direction and in the direction perpendicular thereto according to the quantity of image distortion stored in the first and second memories.
In a second aspect of the invention, in an image forming apparatus, a printer forms an image according to image data in the unit of each pixel, and a sensor detects a quantity of image distortion in the image formed by the printer. Then, a corrector corrects the image by providing print data according to the detected quantity of image distortion, wherein the corrector corrects the image distortion in a predetermined range. When the detected quantity of image distortion exceeds the predetermined range, a controller sets the quantity of image distortion to a maximum in a predetermined range and makes the corrector correct the image according to the corrected quantity of image distortion.
In a third aspect of the invention, in an image forming apparatus, before image delay, an encoder converts input data of an image to second image data having a smaller number of gradation levels than the input data. Then, a delay circuit delays the second image data by a predetermined time. Next, a decoder converts the second image data delayed by the delay circuit to third image data having a larger number of gradation levels than the delayed image data, and a corrector corrects the third image data according to image distortion. Then, a printer performs printing according to the corrected image data.
An advantage of the present invention is that image distortion can be corrected without using a complex structure because the position error of the sensor and image distortion quantities of each color can be obtained at the same time with one kind of sensor by detecting predetermined patterns of the colors.
Another advantage of the present invention is that even if the detection result of the quantity of image distortion exceeds the maximum correctable distortion, the image can be corrected by exploiting the capability of the image distortion correction circuit.
A further advantage of the present invention is that the memory capacity of the delay memory can be decreased.
A still further advantage of the present invention is that the bow correction can be performed at a finer level.